Six months after US Steel dumped a plume of toxic metal into a Lake Michigan tributary, the company quietly reported another spill at the same northwest Indiana plant and asked state environmental regulators to keep it secret, according to newly released documents.

The 56.7 pounds of chromium released in late October by the company's Midwest Plant was 89 percent higher than its water pollution permit allows over 24 hours, U.S. Steel revealed in a letter sent to the Indiana Department of Environmental Management.

A wastewater treatment system at the plant malfunctioned on the morning of Oct. 25, a problem that wasn't noticed until the next day. Indiana officials were notified Oct. 27, according to the company's letter, which is dated Oct. 31 and requested "confidential treatment" of the incident.

Law students at the University of Chicago discovered the letter while tracking pollution violations at U.S. Steel and other factories on the southwest shore of Lake Michigan. The document tops a stack of evidence gathered by attorneys at the university's Abrams Environmental Law Clinic for a lawsuit they are preparing that will accuse the Pittsburgh-based steel giant of repeatedly violating the federal Clean Water Act since 2011.

A review of online press releases shows neither state officials nor the U.S. Environmental Protection Agency informed the public about the latest miscue at the plant, part of a complex of steel mills that divides the Indiana Dunes National Lakeshore in Portage.

An EPA spokeswoman said Indiana officials didn't tell the agency about the spill until Tuesday morning, following inquiries from the Tribune. The Indiana environmental agency said it was preparing a response to questions but nothing had been received by Tuesday evening.

U.S. Steel said it is working with Indiana officials to "ensure there is no environmental impact" from the October spill of wastewater containing suspended particles of chromium. The company declined to comment on the potential lawsuit.

Unlike the previous spill, which occurred in April, U.S. Steel did not report the latest incident to the National Response Center, a warning system overseen by the U.S. Coast Guard to alert local authorities about oil spills and chemical releases, records show.

The spill wasn't serious enough to merit reporting "and did not pose any danger to water supply or human health," a company spokeswoman said.

In April, fishermen spotted a bigger spill in Burns Waterway, a man-made slip that runs along the west edge of the steel mill and drains into Lake Michigan. They called local television stations, drawing emergency responders from the U.S. EPA's Chicago office who confirmed the plume contained hexavalent chromium, a highly toxic version of the metal made infamous by the movie "Erin Brockovich."

Operators of the water supply for the nearby town of Ogden Dunes responded by temporarily shutting off its Lake Michigan intake; Chicago conducted emergency testing of its own water supply; and the National Park Service closed four beaches as a precaution.

U.S. Steel later reported that 346 pounds of chromium had poured out of a rusted pipe into the waterway, including 298 pounds of hexavalent chromium. A month later the company filed another report that estimated substantially more hexavalent chromium ended up in the waterway - 920 pounds - but it dismissed the finding as an "absurd result" from a single water sample.

Based on publicly available documents, it is unclear how much hexavalent chromium was in last month's spill. The high concentrations of total chromium reported by the company came from the same sewer outfall where the April spill occurred, and the pipe is connected directly to equipment that strips hexavalent chromium from wastewater, records show.

With President Donald Trump pushing an industry-friendly agenda that includes deep cuts in funding for environmental enforcement, the pending legal action gets important facts about the U.S. Steel plant on the record, said Mark Templeton, director of the U. of C. law clinic. The federal clean water law allows citizens to challenge companies on their own but requires them to warn companies and regulators 60 days before filing suit.

"We found what appears to be a history of state and federal authorities sitting on or slow-walking enforcement actions in this particular region," Templeton said in an interview. "Most of this information is technically available to the public. It just takes time for people to find it, understand it and use it to hold polluters and the government accountable."

In its Oct. 31 letter to the Indiana environmental agency, U.S. Steel said it plans to retrain its treatment plant operators and tweak its monitoring equipment.

The letter echoed a company statement after the April incident.

"U.S. Steel has made enhancements to the parts of the facility where the failure occurred and is reviewing additional measures it can take to allow for earlier detection of future issues," the company said at the time, adding that it is "committed to the safety of our employees, to the communities in which we operate and to protecting the environment."

Students and law professors at U. of C. began investigating lakefront industries last year for the Chicago chapter of Surfrider, a nonprofit group that pushes for water quality improvements on behalf of an active band of Great Lakes surfers.

Mitch McNeil, chairman of the local group, said a beach near the Portage steel mill and another next to the BP refinery in Whiting are popular with enthusiasts who don wet suits to protect themselves from harsh, frigid conditions when northerly winds make the lower third of Lake Michigan ripe for surfing.

"One of the first things you notice if you surf a lot in this part of the world is the smell of oil and grease coming off the water," said McNeil, who has been riding waves on the lake for a decade. "We got to a point where enough people were talking about skin rashes and other issues that we needed somebody to dig into what's being dumped into the lake."

The Portage plant and five other nearby facilities legally released a combined 1,696 pounds of chromium into Lake Michigan during 2015, according to federal records. Steel mills are major industrial sources of the metal, which is used to make products rust-resistant.

Though the EPA and the National Toxicology Program say hexavalent chromium can cause stomach cancer, the chemical and steel industries have stalled plans for national drinking water standards. Studies show exposure to the metal also increases the risk of reproductive problems, interferes with childhood development and causes liver and kidney damage.

A Chicago drinking water intake off 68th Street is about 20 miles across the lake from the steel mill.

Quarterly testing by the Chicago Department of Water Management shows levels of hexavalent chromium as high as 0.22 parts per billion in treated drinking water this year - 11 times higher than a health goal California officials adopted in 2009. But levels in Chicago and most other U.S. cities are below a controversial regulatory limit California later adopted: 10 parts per billion.

The California Office of Environmental Health Hazard Assessment defines the health goal, 0.02 parts per billion, as an amount that reduces the lifetime risk of developing cancer to a point considered negligible by most scientists and physicians. The state's regulatory limit was adopted based on other considerations, including the added cost of water treatment.

Chromium discharges from U.S. Steel's northwest Indiana facilities have been the subject of years of heated disputes between the company and federal environmental regulators.

It took the EPA until 1977, five years after Congress overwhelmingly approved the Clean Air Act, to secure the first court decision forcing the company's Gary Works to reduce the amount of heavy metals and other waste it dumped into Lake Michigan and the Grand Calumet River. Several similar cases have been brought against U.S. Steel since then.

As recently as 2007, the EPA intervened to prevent the state of Indiana from scrapping or relaxing limits in the water pollution permit for the Gary Works, including restrictions on hexavalent chromium. The steel mill is still the biggest polluter in the Lake Michigan basin.